1. Field of the Invention
The present invention relates to a quadrature modulator suitable for use in digital mobile communications.
2. Description of the Conventional Art
Japanese laid-open patent publication No. 5-75658, for example, discloses a conventional quadrature modulator. The disclosed quadrature modulator has a differential converter converting an input carrier into two carriers that are 180.degree. out of phase with each other, i.e., a positive-phase carrier and a negative-phase carrier. The amplitude ratio of the positive- and negative-phase carriers is controlled by an amplitude ratio control circuit based on a phase difference signal that is detected by a phase difference detector. After the amplitude ratio has been controlled, the positive- and negative-phase carriers are supplied to a phase converter, which converts them into two carriers having a phase difference based on the amplitudes of the positive- and negative-phase carriers whose amplitude ratio has been controlled. The two carriers outputted from the phase converter are then limited in amplitude separately by respective limiters. The phase difference detector then detects the difference between 90.degree. and the phase difference between the amplitude-limited carriers outputted from the phase converter. The detected phase difference outputted from the phase difference detector is supplied to the amplitude ratio control circuit, which controls the amplitude ratio so that the carriers outputted from the phase converter will be 90.degree. out of phase with each other. The carriers outputted from the phase converter as they are limited in amplitude by the limiters are multiplied by sine and cosine components of a baseband signal, and the product output signals are combined with each other.
In the case where the conventional quadrature modulator is implemented by an integrated circuit, any changes in the phase relationship between the carriers, which would otherwise be caused by characteristic variations of circuit components and fluctuations of the constants of circuit components due to temperature changes, are suppressed. It is desirable that the quadrature modulator be not affected by absolute errors of the constants of circuit components and frequency changes of the carriers, and that a phase distortion produced by an amplifier be corrected when the modulated signal is amplified by the amplifier.